Machan did you used solder flux ? I recomend to use some flux (high quality liquid flux)
some additions from me (To SL EK members):
To clean old thermal paste, can use surgical spirit (isopryl alchohol) its easy to find and cheap too.
Don't use Microwave oven (instant kill mb)
I didn't have Liquid Flux yesterday... LOL I yeah forgot to mention NEVER USE MICROWAVE... Gas or electric...If its a small electric oven thats more ideal.....yeah surgical spirit are the same but you will have to wait until it dries completely....